Crystal structure of bis[octakis(dimethyl sulfoxide-κO)ytterbium(III)] pentabromidoplumbate(II) tribromide dimethyl sulfoxide monosolvate: a ytterbium-doped lead halide perovskite precursor

A mixture of PbBr2 and YbBr3·nH2O in a dimethyl sulfoxide (DMSO) solution yielded single crystals of a lead halide perovskite precursor with ytterbium, bis[octakis(dimethyl sulfoxide)ytterbium(III)]pentabromidoplumbate(II) tribromide with dimethyl sulfoxide as co-crystallite. These single crystals react with a caesium chloride solution, exhibiting near-infrared (NIR) luminescence by visible photoexcitation, suggesting the formation of Yb3+-doped lead halide perovskites

A mixture of PbBr 2 and YbBr 3 ÁnH 2 O in a dimethyl sulfoxide (DMSO) solution yielded single crystals of a lead halide perovskite precursor with ytterbium, bis[octakis(dimethyl sulfoxide)ytterbium(III)]pentabromidoplumbate(II) tribromide with dimethyl sulfoxide as co-crystallite, [Yb(C 2 H 6 OS) 8 ][PbBr 5 ] 0.5 -Br 1.5 Á0.5C 2 H 6 OS. The complex ions PbBr 5 3À and Yb(DMSO) 8 3+ are present in the crystal together with three Br À ions and DMSO molecules. X-ray crystallography revealed that the Br À ions in YbBr 3 are replaced by the solvent and bound to a Pb II atom or remain free. The presence of PbBr 5 3À units, which are molecular ions with a square-pyramidal structure, is also observed. These single crystals react with a caesium chloride solution, exhibiting near-infrared (NIR) luminescence by visible photoexcitation, suggesting the formation of Yb 3+ -doped lead halide perovskites (CsPbBr 3-x Cl x ÁYb 3+ ).

Chemical context
Lead halide perovskite crystals have attracted considerable attention in the fields of solar cells and optoelectronics (Lee et al., 2012;Burschka et al., 2013;Fu et al., 2019). Lead halide perovskite crystals have been investigated extensively owing to their facile solution-phase fabrication, high energyconversion efficiency, and characteristic photoresponse. Lead halide perovskites can easily be prepared by spin coating microcrystalline thin films in solution.
Highly polar solvents, such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), are used in fabricating perovskite thin films by solution processing. Typically, these solvents are removed by thermal annealing using a hot plate or air drying after spin coating, and crystal growth proceeds as the solvent becomes supersaturated. The crystal morphology and crystalline phase depend on the annealing temperature and treatment time (Tenailleau et al., 2019;Bi et al., 2014;Xiao et al., 2014;Jung et al., 2019). The morphology of perovskite films, such as the film thickness and grain boundaries, signifi-cantly affects the performance of solar cells. Complex formation between Pb atoms and solvent molecules in the perovskite precursor solution significantly influences the film morphology (Ozaki et al., 2017;Wakamiya et al., 2014;Ozaki et al., 2019). The addition of CH 3 NH 3 I dissolved in 2-propanol to 1D crystals displaced the DMF solvent, forming a 3D perovskite structure. The addition of CH 3 NH 3 I dissolved in 2propanol to these 1D crystals suspended in DMF solvent forms a 3D perovskite structure (Wakamiya et al., 2014). The CH 3 NH 3 I-PbI 2 -DMF intermediate formed by CH 3 NH 3 I addition was also observed during thermal annealing. DMF coordination with the intermediate is thought to be responsible for Ostwald ripening (Guo et al., 2016). Additionally, when DMSO was used as the solvent, a PbI 2 -(DMSO) 2 complex was formed, in which DMSO was more strongly coordinated to PbI 2 than DMF (Miyamae et al., 1980). Lead halide perovskite thin films have been investigated extensively for solar cells and various other fields, including optoelectronics. Recently, the efficient luminescence of rareearth elements using a lead halide perovskite as an optical absorption antenna was reported by doping ytterbium into a 3D CsPbBr x Cl 3-x perovskite (Kroupa et al., 2018;Erickson et al., 2019). However, the crystal structure of lead halide perovskites doped with rare-earth elements and their mechanism of formation remains unclear. In this study, precursor single-crystals of a lead halide perovskite doped with rare-earth elements, bis[octakis(dimethyl sulfoxide)ytterbium(III)] pentabromidoplumbate(II) tribromide dimethyl sulfoxide solvate, were successfully prepared, and the structure of the precursor crystal was determined.

Structural commentary
The obtained structure exhibits an alternating sequence of PbBr 5 3À and 2[Yb(DMSO) 8 ] 3+ units ( Figs. 1 and 2). The [Yb(DMSO) 8 ] 3+ unit is considered to possess three Br À (Br3, Br4) ions as counter-anions. Interestingly, the PbBr 5 3À unit exhibits a square-pyramidal structure. Lead halide compounds often show lead-centered octahedral structures, and there have been no previous reports of the PbBr 5 3À molecular ion with a square-pyramidal geometry. The free atom Br3 is located on the straight line of the Br2-Pb1 bond, and the Pb1Á Á ÁBr3 distance is 6.781 (9) Å (Fig. 1). The free Br3 atom is located at a distance more than twice that of Br2 in the Pb1-Br2 bond [2.814 (4) Å ], suggesting that there is no Pb1-Br3 interaction.
The DMSO molecule as co-crystallite is disordered, and the exact configuration was difficult to determine. Thermogravimetric analysis (TG-DTA) of the crystals revealed a weight loss of 3.4% at approximately 410 K, with an endothermic peak, corresponding to a dissociation of 0.5 equivalents of    2À and free Br À anions in the precursor crystal, with displacement ellipsoids at the 50% probability level. The disordered DMSO molecule is omitted for clarity. Symmetry codes: (i) y, 1 2 À x, z; (ii) 1 2 À y, x, z; (iii) 1 2 À x, 1 2 À y, z; (iv) 3 2 À x, 1 2 À y, z.
DMSO relative to Yb (theoretical value 3.1 wt%) (Fig. 3). The crystal structure resembles that of a 1D perovskite with a series of (PbX 5 3À ) units (Wang et al., 1995). However, the weak interactions between the Br À ions and DMSO molecules in the gaps between the (PbX 5 3À ) units prevents the 1D perovskite from bridging. All halogen ions were lost when YbBr 3 was added, and DMSO is coordinated to the Yb III atom instead. Several Br À ions react with PbBr 2 to form PbBr 5 3À , and therefore YbBr 3 has served as a source of halogen ions in the lead halide perovskite framework.

Photophysical analysis
The precursor crystal did not exhibit any luminescence upon irradiation with visible light. In contrast, the dropwise addition of a methanol solution containing caesium chloride to the precursor crystals, followed by annealing at 473 K for 5 min, resulted in the formation of light-yellow microcrystals. The microcrystals exhibited Yb 3+ -derived near-infrared (NIR) emission at 980 nm upon photoexcitation at 400 nm (Fig. 4). This indicates that the precursor crystals reacted with caesium chloride, and Yb 3+ -doped 3D lead halide perovskite crystals (CsPbBr 3-x Cl x ÁYb 3+ ) (Erickson et al., 2019) were formed. The NIR luminescence of doped Yb 3+ was observed, in addition to the visible-light absorption of the lead halide perovskite crystals.

Synthesis and crystallization
PbBr 2 and YbBr 3 ÁnH 2 O were dissolved in DMSO (anhydrous, Fujifilm Wako Pure Chemicals) to prepare a 0.5 M solution. The solution was heated to 373 K using a hot plate; acetone was added gradually to obtain colourless needle-like crystals (Fig. 5).

Refinement
The crystal data, data collection, and structural refinement details are summarized in Table 1. Because the precursor crystals contain numerous heavy atoms, it was difficult to analyze the residual electrons of these atoms; therefore, an empirical absorption correction using spherical harmonics was applied. The residual electron densities Á max and Á min of 8.97 and À1.78 e Å À3 are located 0.912 and 0.918 Å , respectively, from the Pd atom. Near-infrared emission from Yb 3+ after treatment of the precursor crystals with a CsCl methanol solution.   olex2.solve (Bourhis et al., 2015); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015); molecular graphics: Olex2 1.5 (Dolomanov et al., 2009); software used to prepare material for publication: Olex2 1.5 (Dolomanov et al., 2009).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (